Satellite antennas may be split into two large classes: single-beam and multi-beam.
Considering single-beam antennas, a typical target is the development of systems able to scan high gain antenna patterns over limited sectors of space (Limited Field Of View-LFOV-Applications). LFOV applications range from telecommunication satellites (e.g., Low Earth Orbit dynamically reconfigurable coverage), satellite remote sensing (e.g., ScanSARs, Spotlight SARs, Scan On Receive technique) and ground systems (e.g., radars). In scanning systems, the beam pointing directions may be modified essentially in two different ways: either by a mechanical movement or rotation of the antenna or by an electronic scanning.
Typical antennas requiring physical movements for adjusting the beam pointing direction are passive Reflectors, while Direct Radiating Arrays (DRAs) or Array Fed Reflectors (AFRs) represent effective antenna configurations for employing the electronic beam scanning.
DRAs consist of linear or planar arrangements of radiating antenna elements (AEs), while AFRs are constituted by one or more reflectors fed by a linear or planar antenna array. In both DRAs and AFRs, the beam pointing is obtained by electronically varying the phases and/or the amplitudes of the antenna elements constituting the array. This is performed using a beam-forming network (BFN) comprising fixed and/or adjustable weight elements (WEs) i.e. phase shifters and variable attenuators, power dividers and/or combiners, etc.
On the other side, considering multi-beam antennas, one of the most important goals consists in using a multi-beam coverage in order to obtain high gain and support spatial frequency reuse to save power and increase the throughput. Typical multi-beam applications comprise telecommunication satellites generating multi-beam coverage's and ground systems (e.g., ground vehicular and aeronautical telecommunication terminals, antennas for mobile systems base stations, etc.). Like their single-beam counterparts, multi-beam antenna can use passive reflectors or arrays (DRAs, AFRs) provided with suitable BFNs.
In conventional Active Arrays, the number of antenna elements (AEs) coincides exactly with the number of required Weight Elements (WEs). If the antenna is used in transmission, this also coincides with the number of High-Power Amplifiers (HPAs), since each antenna element is provided with a WE (more precisely, one WE per AE is required for each beam to be generated) and a HPA. This implies that the excitation of each AE is controlled by one amplifier and one WE, (together referred to as a Control Element—CE).
DRAs present the most attractive solution among the different antenna configurations in terms of high scanning flexibility and reconfigurability. However, they comprise a much higher number of AEs compared to AFRs and reflectors, resulting in highly complex and bulky BFNs in order to realize the proper set of excitations. This is particularly true in multi-beam applications and when high reconfigurability is required.
The BFN complexity increases with the number of beams NB and Antenna Elements (AEs) NE. The complexity further increases when beam shape and pointing reconfigurability are required. Indeed, full flexibility would be reached if any beam signal could be independently addressed to any Antenna Element with full freedom of phase and amplitude weighting. This would require NB×NE active Weight Elements.
The placement of the amplifiers before or after the BFN (considering a transmit antenna) depends on the losses and power handling characteristics of the BFN itself. In particular, when the beam signals are single-carrier and/or when NB<<NE it would be preferable to have an amplifier per beam. However, this is not possible if the BFN introduces significant losses.
Basically, there are two sources of losses in any practical BFN design: theoretical and implementation losses. Implementation losses are mainly ohmic, depend on the characteristics of the devices composing the BFN and can be mitigated through accurate device design and by limiting the network complexity. “Theoretical losses” are more fundamental. They derive directly from the Microwave Circuit Theory and may be avoided only imposing precise constraints on the Scattering Matrix of the BFN, which would limit the degrees of freedom in the radiation pattern synthesis, affecting the final radiating performance of the antenna. As a consequence, most BFN are not “lossless” (more precisely: they have theoretical losses in addition to implementation losses). This prevents the use of single-amplifier-per-beam configurations with passive reconfigurable multi-beam BFNs, in favour of active array configurations where the number of amplifiers is equal to the number of antenna elements—see FIG. 1A showing an array AR of antenna element AE, each connected to an individual HPA, all the HPAs being fed by a reconfigurable beam-forming network R-BFN.
Several methods have been proposed in order to reduce the complexity of BFNs, particularly for DRAs. A first approach consists in the use of non regular (sparse) layouts for DRAs and active constrained lens antennas in order to minimize the number of AEs, and therefore of WEs and HPAs ([1], [2]).
Another design method consists in decomposing the array of NE antenna elements in a number of subarrays NSA (where NSA<<NE) composed of groups of antenna elements see FIG. 1B. For manufacturability purposes (modularity and scalability) all the sub-arrays are most often exactly identical (composed of NESA antenna element) and translationally arranged on a regular Sub-Array Lattice (SAL).
A reconfigurable BFN of an array antenna based on the sub-arraying concept can be advantageously decomposed in a cascade of two simpler BFNs:                a reconfigurable BFN (R-BFN) comprising NB×NSA active weight elements; and        a fixed passive BFN distributing the signal from each output port of the reconfigurable BFN to a group of Antenna Elements forming a respective Sub-Array (SA).        
The sub-arrays can be either disjoined (i.e. non-overlapping—NOSA)—refer to FIG. 1B, or overlapping (OSA), in which case at least some antenna elements belong to several sub-arrays at a time—refer to FIG. 2A and FIG. 2B.
In the Non-Overlapping Sub-Arrays case, the fixed passive BFN reduces to a plurality of identical 1:NESA Single Mode Networks (SMNs)—typically comprising only hybrid couplers and fixed phase shifters. Considering that these networks can be realized fulfilling ideal matching and lossless conditions, the HPAs can be moved from the Antenna Elements to the input of the SNMs, thus reducing their number from NE to NESA with evident impact on system cost and complexity (FIG. 1B, where the lossless non-reconfigurable BFN feeding the sub-arrays is designated by reference NOSA-BFN). Unfortunately Non-Overlapped Sub-Array solutions fall short in pattern control and find applications only when severe degradations in sidelobe and grating lobes levels can be accepted.
To overcome the limitations in pattern control typical of Non-Overlapping Sub-Array based architectures, the concept of Overlapping Sub-Arrays (OSA) has been introduced. FIGS. 2A and 2B show OSA architectures with HPAs disposed at the Antenna Elements inputs (2A) and at the output port of the reconfigurable BFN (2B); in the first case, the fixed BFN (OSA-BFN) can be lossy, while the latter case implies that the fixed BFN (OSA-BFN′) is lossless.
Some authors investigated the minimum number of control elements achievable using sub-array technique for both linear and planar periodic arrays [1], [4], [5], [6]. Other authors focused their work on the development of appropriated feeding networks aimed at achieving the theoretical goals with low-complexity realizations. A summary of these networks has been presented by Skobelev [6].
Interesting network designs have been carried out by Mailloux et alii [8], [9], [10], [11], [12], [13], DuFort [14], Skobelev [15] and Shelton [17]. More particularly:                Skobelev [15] analysed a lossless network consisting in power dividers and directional couplers, whose coupling coefficients may be derived by an optimization process. The network, also known as “chessboard network”, presents the advantage to be lossless from the point of view of Microwave Circuit Theory. Reference [16] discusses improvements in the radiative performances of the “chessboard network”.        Shelton [17] introduced the concept of Double Transform Network, which consists in using systems able to perform a double Fourier Transform in order to obtain a flat-topped sub-array radiation pattern. Such systems may be realized with constrained networks, lenses and also digitally, preserving the lossless property of the system. Application of the Double Transform method to on-board satellite arrays has been reported in [1].        DuFort contribution [14] represents another important milestone in the design of beamforming networks for Overlapped Sub-Arrays. It proposes a BFN composed of matched hybrids arranged in cascade, whose coupling coefficients have to be determined in order to approximate an ideal OSA radiation pattern.        Another interesting contribution to Overlapped Sub-Array techniques is due to Ploussios [19] who designed a BFN architecture valid for both LFOV and multi-beam applications, and also presented an extension to planar arrays.        Herd [20] proposed an hybrid (analogue/digital) architecture, which is applicable to planar array, however, the BFN doesn't fulfil the lossless property and cannot be used for high power applications.        Craig and Stirland [21] introduced several embodiments of a new BFN architecture and an optimization method to set the design parameters (distance between radiators, distance between adjacent control points, total number of array radiators and control points . . . ) starting directly from the requirements in terms of radiation properties (Beam-Width, Field Of View, etc.). The proposed BFN may be digital, analogue or hybrid (realized with a combination of digital and analogue devices).        
All the approaches mentioned above suffer from at least one of the following drawbacks when used in practical applications:                Most prior art BFNs (e.g. [14], [15], [17]) are only designed for linear arrays, while most of the practical applications require the use of planar arrays;        Some prior art BFNs (e.g. [20], [21]) are not lossless from the point of view of the Microwave Circuit Theory, which forces to place the HPAs at the input ports of the antenna elements, and therefore to use NE of them. Another drawback, in case of HPAs placed at the input ports of the antenna elements, consists in the fact that signals at OSA-OPs may be characterized by different amplitudes (which are set by the BFN), implying that the HPAs have to be operated at a certain Input Power Back-Off (IBO) with respect to saturation. The associated HPA low DC-to-RF power conversion efficiencies, high dissipated powers and need for improved thermal dissipation system put additional stringent constraints to payload feasibility.        Other prior art BFNs are theoretically lossless but exhibit high complexity (e.g. [14], [15], [19]), and therefore non-negligible implementation losses.        